A field device is, in the normal case, an intelligent unit, composed of a sensor, which detects the pertinent process variable, and an integrated, or spatially remote, measurement transmitter, which makes available information concerning the process variable.
Often applied in process automation technology are field devices serving for registering and/or influencing process variables. Examples of such field devices include fill-level measuring devices, mass flow measuring devices, pressure- and temperature-measuring devices, pH-redox measuring devices, and conductivity measuring devices, which register the corresponding process-variables, fill-level, flow, e.g. flow rate, pressure, temperature, pH-value, conductivity, etc. Field devices for the most varied of applications are manufactured and sold by the Endress+Hauser.
Frequently, field devices are connected via a so-called fieldbus with a superordinated unit, e.g. a control station or a process control system. The superordinated control station serves for process visualizing, for process monitoring, for process control, as well as for servicing the field devices connected to the fieldbus. Examples of such fieldbusses include: CAN, CAN-OPEN, HART, Profibus PA, Profibus DP, Profibus FMS, Foundation fieldbus.
With the help of a fieldbus, it is possible, not only to transmit measured values of a field device to the central control unit, or control station. Thus, besides pure measured value transmission, field devices also enable transmission of various other kinds of information stored in the field device, information, such as, e.g., parameter-information (zero-point, measured value range, etc.), measurement curves and diagnostic information. Furthermore, the field devices are remotely serviced by the control station. For this, special hardware- and software-components are necessary, both in the field device as well as also in the control station.
In process control technology, development is moving in the direction of implementing in the field device always more functionalities, which relate, directly or indirectly, to the actual measuring- and monitoring-task of the field device; especially, a field device must be able to cover all special demands, which a customer has. These varied functionalities are performed by at least one microprocessor arranged in the field device. This microprocessor is, naturally, also responsible for its original purpose, i.e. for making available information concerning the process variable to be ascertained and monitored.
The growth of the functionality moved into the field device means, on the one hand, for the device manufacturer, an increased manufacturing- and developmental effort; and, for the user, it means, on the other hand, an increased administrative effort. Thus, for both participants, higher costs arise. Especially, the associated schooling effort and the number and size of handbooks become quite significant. Yet, it is to be noted, that an individual user needs, as a rule, usually only a fraction of the available functionalities of a field device for the user's particular application.
Disclosed in DE 103 25 277 A1 is a variable field device for process control technology. In this case, the device is connected to an external communications medium for the purpose of data transmission, and its functionalities are adaptable application-specifically. While the field device still only provides the basic functions, e.g. measured value production, application-specific functionalities, such as e.g. providing frequency/pulse outputs, are implemented by separate functional units, with the field devices and the separate functional units exchanging data via the communications medium. The communications medium is e.g. a two-wire line. The functional unit is, for example, an externally connectable, operating, or servicing, tool.
The solution of the state of the art makes the field device very variable and application-specifically adaptable. Especially, it has only the functionalities that it needs. Also, servicing such a field device is exceptionally easy, since it has only the user-required functions. Furthermore, transparency of the measured-value further-processing is provided for the user. The field device delivers a measured value, which is further-processed in the appropriate functional units. Through its simple construction, the variable field device is very cost-favorable as regards its manufacture and handling.